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01 September, 2007
Sunspot numbers pulled up from 0 this week, but barely. Average daily
sunspot numbers rose over nine points to 12.9. Geomagnetic conditions
were quiet. Conditions have been quiet for so long that we may not appreciate
this, as many of us wish for more solar activity and sunspots.
It wasn't long ago when we had more sunspots, but the accompanying geomagnetic
activity made conditions difficult. Check the Planetary A index in October
2002, April, June, September and December 2003, and into January 2004.
For 2003, check http://www.sec.noaa.gov/ftpdir/indices/
old_indices/2003_DGD.txt.
For any other year, just change 2003 toward the end of the URL, and
to check sunspot numbers change DGD to DSD at the end of the URL.
If you read ARRL Propagation Bulletins from back then, in addition to
complaints about the higher geomagnetic activity, you'll find stories
of better VHF propagation during periods of geomagnetic disturbance. You
can see old bulletins back through 1995 at, http://www.arrl.org/
w1aw/prop/.
This morning the Australian Ionospheric Prediction Service sent out a
warning of increased geomagnetic activity centered on September 1 due
to a wind stream from a solar coronal hole. They predict today, August
31, will be quiet with increasing activity late in the day, unsettled
to active conditions with possible minor storm on Saturday, September
1, and mostly unsettled conditions September 2.
Geophysical Institute Prague predicted earlier that August 31 would be
quiet to unsettled, September 1 unsettled to active, unsettled conditions
September 2-3, quiet September 4-5, and unsettled to active again on September
6.
Over the same period the US Air Force predicts a Planetary A index of
15, 25, 12, 12, 8, 5 and 15 for August 31 through September 6. From the
same prediction, it looks like September 8-17 may see a return of 0 sunspot
days.
In response to last week's mention of ionospheric sounders, Pat Dyer,
WA5IYX of San Antonio, Texas wrote in with some useful information and
links.
At http://digisonde.haystack.edu/Search.html
you can view recent ionographs from the MIT Haystack Observatory at Millstone
Hill in Westford, Massachusetts. In the typical Lowell Digisonde graph,
we can see echoes from the ionosphere over a range of 1-10 MHz, for radio
energy beamed straight up and swept through the radio spectrum. Frequency
is shown along the bottom, on the x-axis, and the vertical axis is calibrated
in kilometers. This is the calculated height of the part of the ionosphere
that is returning echoes.
On the left side at the top we see foF2, which is the approximate frequency
of the strongest echoes. Along the bottom, below the graph, is a list
of "D" and "MUF." This is the calculated Maximum Usable
Frequency for the patch of ionosphere over the observatory, related to
distance. So stations 1000 km apart, with the observatory in the middle,
would see the MUF value associated with that distance reflected by the
patch of sky above the observatory.
Currently I am looking at a chart for August 31 at 0730z. For the distances
of 100, 200, 400, 600, 800, 1000, 1500 and 3000 km I am seeing MUF values
of 2.7, 2.7, 2.8, 3.0, 3.1, 3.4, 4.3 and 6.4 MHz. This means that stations
1500 km apart, or 750 km on each side of the observatory, could communicate
well on 80 meters, but not at 1000 km or shorter distances.
WA5IYX gives an example at,
http://www.qsl.net/wa5iyx/ionogram/01701400.gif.
Referring to the example, Pat writes, "The x-axis is the swept ionosonde
freq (MHz) of the pulse sent straight up (VI, vertical incidence) with
(if any) echoes displayed.
"The short red dashes just under the x-axis are freq segments where
their ionosonde is shut off to avoid QRMing the existent services there."
He continues, "The y-axis is the "virtual height" in km
as
determined by the ms time delay for the echo (1 ms round trip is approximately
300/2 km). It's called virtual since (with the exception of Es) the reflection
takes place over a depth of 10s of km, with the pulse actually being slowed
down just before and after. This is what gives those striking curves to
the F-region traces.
For F1 it's the lower E region and for F2 the F1 - they are still
slowing (retarding) down the pulse, less so as the freq rises,
giving the appearance that the F2 layer is at first high, then
lowers, and then rises again. Only the latter is really indicative of
a true height change as the pulse goes higher (deeper) into the layer
before being reflected as the freq is raised (and eventually fully penetrating
it)."
"foF2 is the highest VI freq reflection of the ordinary wave from
the F2 region. A plasma (which is what the ionosphere is) in a magnetic
field splits an incident radio wave into two components (birefringence)
- the ordinary (o) and extraordinary (x) wave. On VI ionograms these echoes
are separated by the natural gyrofrequency (the spin/sec that a free electron
exhibits in a given strength magnetic field). So that separation will
vary across the earth as the magnetic field (in long-lat as well as height).
For Lowell it's about 1 MHz difference."
Pat goes on to say, "In my Lowell sample, that 27.58 MUF is the
calculated maximum oblique frequency that sampled portion of the F-layer
should be able to return (in their case over a 3000 km range). These are
calculated from the Secant Law using the foF2 and F2 values. Their current
display format lists (at the bottom left) the interpolated MUFs for ranges
of 100, 200, 400, 600, 800, 1000, 1500, and 3000 km. (This isn't visible
in his example, but the actual MUFs not shown are 4.4, 4.4, 4.6, 4.8,
5.2, 5.7, 7.3 and 11.2 MHz)."
He closes by saying, "A super-complex explanation of Dynasondes
is at, http://www.ngdc.noaa.gov/stp/IONO/Dynasonde/
tutorial/tutorial.html."
Thanks, Pat!
If you would like to make a comment or have a tip for our readers, email
the author at, k7ra@arrl.net.
For more information concerning radio propagation, see the ARRL Technical
Information Service at,
http://www.arrl.org/tis/info/propagation.html.
For a detailed explanation of the numbers used in this bulletin, see,
http://www.arrl.org/tis/info/k9la-prop.html.
An archive of past propagation bulletins is at, http://www.arrl.org/w1aw/prop/.
Monthly propagation charts between four USA regions and twelve overseas
locations are at, http://www.arrl.org/qst/propcharts/.
Sunspot numbers for August 23 through 29 were 12, 12, 14, 13, 12, 14
and 13 with a mean of 12.9. 10.7 cm flux was 70.8, 71.6, 71.5, 70.1, 69.2,
70.1, and 69.6 with a mean of 70.4. Estimated planetary A indices were
2, 2, 8, 10, 12, 11 and 4 with a mean of 7. Estimated mid-latitude A indices
were 1, 1, 6, 10, 10, 9 and 4, with a mean of 5.9.
Source: The
American Radio Relay League
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